Heretofore, in order to improve the quality of glass products, a refining step of removing bubbles formed in molten glass is carried out prior to forming in a forming apparatus of molten glass formed by melting glass materials in a melting tank.
As the refining step, a method has been known wherein a refining agent such as sodium sulfate (Na2SO4) is preliminarily blended with the materials, and molten glass obtained by melting the materials is stored and held at a predetermined temperature for a certain time, whereby bubbles in the molten glass are made to grow and float up by the refining agent and are removed. Further, a vacuum degassing method has also been known wherein molten glass is passed through a vacuum degassing vessel, the interior of which is maintained in a predetermined degree of vacuum, whereby bubbles included in the molten glass are made to grow in a relatively short time, and the bubbles are made to float up to the surface of the molten glass employing buoyancy of the grown bubbles, and the bubbles are broken at the surface of the molten glass, whereby the bubbles are removed from the molten glass.
In order to efficiently remove the bubbles from the molten glass, it is preferred to carry out the above two methods in combination, that is, to carry out the vacuum-degassing method using molten glass having a refining agent added thereto.
In order to effectively and securely remove the bubbles in molten glass, a process is necessary such that the bubbles are made to grow in the molten glass and float up to the molten glass surface and are broken, and in order to carry out such a process securely and effectively, it is required to maintain the degree of vacuum in the vacuum-degassing vessel within an appropriate range.
In order to effectively remove bubbles from molten glass, it is necessary that gas components dissolved in the molten glass enter into the bubbles, whereby the bubbles continuously grow. In order to cause such a phenomenon that bubbles continuously grow, it is necessary to reduce the pressure to a pressure of at most a certain threshold pressure. Gas components greatly contribute to growth of the bubbles in the vacuum-degassing step, are H2O gas formed from moisture in the molten glass and SO2 gas formed by decomposition of SO3 dissolved in the molten glass. Thus, the present applicant has disclosed that the bubbles in the molten glass can effectively be removed by adjusting the pressure in the vacuum degassing vessel to be lower than the bubble growth starting pressure determined by the moisture concentration (β-OH value) in the glass, the SO3 concentration and the temperature of the molten glass (Patent Document 1).
It has been known that in molten glass after melting or after refinement, bubbles form also at the interface between the molten glass and platinum or bricks as a structure (Patent Document 2). Patent Document 2 discloses that formation of bubbles can be suppressed by defining the content of SO3 in the molten glass within a predetermined range.